In a WDM optical communication system, client signals are transmitted at different carrier wavelengths. The client signals are combined into a WDM signal using a multiplexer and transmitted along a single optic fiber. The WDM signal is typically composed of several channels of information with each channel corresponding to one of the client signals. When the WDM signal is received by the receiver terminal, the individual channels are separated by a demultiplexer and processed by optical receivers.
By partitioning and maintaining multiple wavelengths in a single fiber link, WDM technology can provide extended transport capacity and is an obvious fit for transmitting high volumes of traffic. This is particularly true for long distance communications schemes. As the demand for bandwidth is continuously rising, current WDM systems are now designed to provide even more bandwidth capacity.
Current WDM systems are not always designed solely with greater transport capacity. In some cases, the transport capacity provided by currently available WDM systems is adequate. When bandwidth is abundant, cost often becomes the prime consideration. In metropolitan environments, for example, the bandwidth requirements of local systems are typically lower than those of long distance WDM networks and therefore it is often desirable to reduce the initial cost of deploying a WDM network albeit at the expense of bandwidth.
In addition to greater bandwidth capacity, WDM systems are often designed with other attributes such as, for example, bidirectional operation and/or increased reliability to reduce link failures. However, these requirements does not permit a cost-effective and efficient allocation of the resources present in the network.
In conventional unidirectional WDM systems for example, the installation of at least two fiber links is required to achieve transmit and receive operations. Additional fibers are also necessary in both unidirectional and bidirectional WDM systems to “protect” the working fibers in the event of a link failure. Current protection configurations which require the installation of additional fiber links between nodes include dedicated protection (1 protection fiber for each fiber link also referred to as 1:1 protection), shared protection (1 protection fiber for N fiber links or 1:N protection) and ring protection. Therefore, in order to reduce the initial cost of a WDM optical network, it may be desirable to reduce the number of fiber connections used.
In addition to this ineffective use of fibers, the accommodation of multiple fiber links necessitates replicating some of the filtering equipment required at each node. In particular, the installation of a unidirectional or bidirectional WDM system such as those described above requires duplicating some of the filtering apparatus for each fiber used. The equipment unique to each fiber link and present at each node typically includes an add-drop multiplexer (ADM) which itself is comprised of a series of multi staged narrowband filters with high figures of merit (FOM). ADMs are typically used to achieve communication between nodes along a main fiber. Briefly described, an ADM is used in a WDM network to filter out and reroute at least one channel of the WDM signal (hereinafter the “drop channels”) while the remaining channels (hereinafter the “through channels”) travel through the ADM. The ADM can also be used to “add channels” to the WDM signal, using wavelengths that have been vacated as a result of channels being dropped at the ADM or at ADMs earlier in the transmission path. As more than one channel usually needs to be accessed at a network node, multichannel ADMs are used and are typically designed with cascaded filters such that each ADM filter is adding or dropping a channel to or from the WDM signal. The multichannel ADMs contribute significantly to the cost of a WDM optical network, particularly in the case of unidirectional local area networks where numerous multichannel ADMs are used. Duplicating this filtering equipment for each fiber link may prove to have a major impact on the initial cost of the WDM network.
In bidirectional networks, more complex multichannel ADMs are required which also has a substantial bearing on cost. In particular, for a given channel count, the multichannel ADMs used in bidirectional systems have twice the number of cascaded filters per fiber than the multichannel ADMs used in unidirectional networks. This higher number of filters results in higher optical loss on each fiber and necessitates optical amplifiers, thus also considerably increasing the system cost.